Peter Henley/SEATTLE

The dilemma which Boeing faced when launching its Next Generation 737 was whether to update the proven model or start afresh. The big 737 operators wanted an updated 737 for fleet commonality, but they demanded a version which would fly faster, higher and more economically. They also wanted a level of similarity with their existing 737s which would persuade the US Federal Aviation Administration and the European Joint Aviation Authorities to allow common type-ratings, so that pilots could switch between the old and the new without constraint. As if this dilemma was not difficult enough, it was turned into a "trilemma" by those Boeing customers which had invested in the latest-technology 777, and wanted the smaller aeroplane to have commonality with that aircraft.

On balance, the choice pointed to an evolved, improved, 737 rather than a new aircraft, and the eventual result was the Next Generation family of 737s. All three of these (the 737-600/700/800) share a larger wing - increased in both span and chord - greater fuel capacity and CFM International's latest CFM56-7 engines.

The 737-700 is equivalent in size to the 737-300, with 128 to 149 seats. It is undergoing its certification flight-test programme at Seattle, and is due for certification and the start of deliveries to customers in the third quarter of this year. Despite the considerable pressures of the development programme, Boeing invited Flight International to fly the -700 late in July, before certification. The aeroplane available on the day was a demonstrator already designated for eventual delivery to a customer. My mentor for the day was Mike Hewett - engineering test pilot and 737 chief pilot.

The first evidence of Boeing's "design for compromise" is revealed by the cockpit concept. Here Boeing has used modern technology, paradoxically to keep the cockpit as close to the older 737 as possible rather than replacing the old with the new. As Hewett succinctly puts it, the established 737 airlines wanted "-the cockpit door firmly shut" when the designers looked about the existing aeroplane deciding where to make changes. Such inflexibility proved impossible, of course, but many of the "traditional" controls and panels were successfully retained even where the systems lurking behind them were radically different.


Feel at home

The philosophy became one of keeping the old, where possible, to make the occupants feel at home, but, where a control panel could not be retained, its replacement was deliberately made as different as possible so that no inadvertent errors under pressure were likely to be made by 737 "old hands".

Thus the -700 pilot will do similar cockpit checks (for example, switching off the auxiliary power unit (APU) during the climb, even though shutting down the unit could easily have been made automatic), but, should a check be overlooked, the -700's "smart" technology will do it anyway at an appropriate moment. Perhaps the most obvious example of keeping the old to control the new lies in the use of power levers (throttles) which look like the time-honoured 737 items, but, instead of being connected in the centre console to a system of cables and levers, are now joined to the electrical loom of a computer-controlled system.

The most dramatic use of modern technology's inherent flexibility lies, however, in the choice of instrument displays. Here, the Honeywell flat-panel display panels can be fed through the new ARINC 700 standard avionics, with a choice of display format which can mimic either the existing -300 displays or those of the 777. An operator therefore can choose the display which matches that of its existing fleet, but could change its mind as its fleet composition changes, or if it wishes to sell or lease the aircraft. The display change depends only on software and takes about 20min to make at a cost about $10,000 .

The aeroplane we were to ßy had already been painted, trimmed and furnished, so that it was representative of an in-service aircraft. The cabin had an attractive light and spacious look, and had been fitted with the latest design of Boeing overhead luggage bins and ceiling panels, with new, but simple, hand rails moulded beneath the bin lids. The forward portion had first- or business-class seating and the remainder, behind an easily moved dividing panel, had six- abreast economy seating.

The cockpit looks typically 737. The only exception is in the five Honeywell flat panels, two for each pilot's flying displays and a central one for engine displays, while a sixth screen is fitted in the forward part of the centre console (for centre-panel redundancy, or for systems synoptics should a customer so specify).

The overall impression is more of the traditional, less of the modern. The clean, business- like, lines of a modern, freshly designed, cockpit are inevitably not there. The overhead panel looks decidedly "period", with its numerous toggle switches and a distinct lack of push-button control switches or mimic diagrams of relevant systems - these days so much a part of an airline pilot's working environment.

The control wheels are classic Boeing, with their dual-pole electric trim switches and squeeze-to-transmit switches, even down to a small panel with a clip for approach plates and a printed checklist for the take-off, after take-off, descent and landing checks. The pilots' seats are also typical (and excellent) Boeing bought-in items, with fore and aft movement and a swing outboard at the rear limit of travel to ease access to the seat. They have the usual vertical movement, back-recline, adjustment, adjustable and stowable armrests and five-point harnesses. A calibrated panel on the cockpit floor helps the occupant always able to return the seat to the position which suits him or her best; there are no eye-position indicators on the centre windscreen pillar.

The rudder pedals incorporate a rocking function for wheel-brake operation and are adjustable for reach via a small crank handle beneath the instrument panel between each pilot's legs. Outboard of each seat there is ample stowage for a flight bag. Beside each pilot, easily to hand, is an emergency oxygen mask, and above each side window an emergency escape rope is housed in a roof panel. The fuse panels are on the rear cockpit walls and there are two jump seats, one fixed behind the captain's seat, and another which folds out and locks across the cockpit doorframe when required.

The field of view is good, and made even better in steeply banked turns by the two small ancillary windows above the main side windows. Each pilot can easily see the wingtip on his/her side of the aircraft - probably more easily than on earlier 737s because of the latest version's greater wing span - 34.4m instead of 28.7m. The extent of Boeing's "cloning" was revealed when Hewett explained that the -700's flap settings and recommended approach speeds had been chosen to give an aircraft pitch attitude on the approach which matches that of the earlier 737s and thus give the pilots the same runway perspective on finals.


Instrument format

This particular -700 had the instrument format to represent the earlier aircraft. The six basic instruments represented are: the attitude indicator; the horizontal situation indicator; Mach/airspeed indicator (with analogue airspeed in knots and airspeed and Mach number digitally in the centre of the dial); altimeter (analogue with a digital reading and subscale in the centre of the dial); and an analogue vertical-speed indicator (VSI). The presentation of these six instruments initially appears odd because four of them are grouped on the pilot's left-hand display, while the altimeter and VSI are shown one above the other on the left-hand half of the right-hand display.

The other half of this display (which is divided by a vertical line) carries the navigation (or map) display. The pilot's instrument scan therefore has to embrace both panels, but, in practice, this proves not to be a problem; the eye soon adjusts to scanning both panels as if the piece of blank instrument panel between them were not there. If, however, a screen were to fail, the whole display of six instruments would be shown on the one remaining serviceable screen - albeit on a smaller, compact, scale which would be too small for normal use. The electronic flight-instrumentation control panels - one at each outboard end of the central glare shield panel - are the same as those fitted to the earlier 737s.

The two 89kN (20,000lb)-thrust CFM56-7 engines have full-authority digital engine-control (FADEC) - although not all that authority is used on the -700, so that, once again, it is acceptably compatible with the earlier, non-FADEC aeroplanes. The primary and secondary engine indications are shown on the central display panel, but in a format directly akin to the electro-mechanical displays of the earlier versions. The parameters covered are N1 (fan RPM); exhaust gas temperature (EGT); N2 (high-pressure turbine RPM) and fuel flow/fuel used. At the bottom of the primary panel, fuel contents are shown for the three tanks: left, centre and right.

On the secondary panel, oil pressure and temperature, engine vibration and the two hydraulic systems' pressures are displayed. A possible penalty for perpetuating this original 737 layout of engine instrumentation is that there are arguably insufficiently compelling visual cues to direct the attention of pilots under stress to the correct engine to shut down in an emergency associated with engine failure.

While left-hand engine data are displayed on the left-hand column of the two primary engine indicators and that for the right engine is displayed on the right-hand column, these columns are to the left of the display panel, and both the secondary columns are to the right. Modern cockpit philosophy usually strives to ensure that "left is left" and "right is right", to avoid any possible confusion under duress.


Entering the flightplan

The weather at Boeing Field, Seattle, on the day of the flight was dry and bright and became even better as the day wore on. "Our" -700 stood, distinguishable from other 737s only to an educated eye, through the greater wingspan, taller fin and wider tailplane (the tail growth being necessary to counter the greater thrust of the latest engines). In addition to being bigger, the latest wing has vortillons on its outer leading edges to preserve airflow outboard at high angles of attack and to induce the wing to stall first in the inboard area. Ground power was already plugged in when we boarded.

Before starting engines, the flightplan was entered in the flight-management system (FMS) via a control display unit on the forward part of the centre console. The plot was to climb to 41,000ft (14,350m) for high-altitude handling, to carry out stalls and simulated asymmetric work at medium levels, in a designated training area over the centre of Washington State, and to visit Moses Lake Airport for an approach and visual circuits.

The AlliedSignal APU is in a self-contained fireproof compartment in the tail. It can be started and used on the ground and in the air up to the -700's maximum operating altitude. APU starting is an automatic process initiated by selecting START on the forward overhead panel. Any failure in the start cycle leads to automatic shut- down and, once in use, the APU is automatically monitored by an electronic control unit. The engine-start switches are also in the forward overhead panel, and the engine-start levers are in the lower part of the power-lever quadrant.

Once engine rotation has been initiated with the start switch, N2 rotation is noted, then the start lever is moved from OFF to the IDLE detent at 25% N2 RPM. Fuel flow and EGT have to be monitored during the start and the start switch has to be checked OFF at 56% RPM. In this starting process, the full potential of the FADEC is not utilised, to align the procedure with the earlier aeroplanes and engines. On departure from Boeing Field (elevation 50ft and temperature +18íC), the take-off weight was 53,620kg against a maximum permitted 60,345kg. Although the aeroplane was admittedly lighter than its maximum, the rate of climb after take-off was surprisingly good at 4,000ft/min (20m/s) at 1,000ft above ground level, gear and flaps up. For once, an unrestricted climb was allowed by air traffic, resulting in a time to 41,000 ft in 18min 30s.


No mach buffet

The aeroplane was manoeuvred at Mach 0.82, at which speed there was no trace of Mach buffet, and a 30í bank turn was flown manually at an estimated+1.3G, which provoked a light stall buffet, throughout which the handling remained comfortingly crisp. (The buffet boundaries for the 737-700 have not yet been fully established and consequently are not yet published).

Even at this level, a stick "slap" to disturb the elevators and a light rudder "doublet" were countered with a nearly deadbeat performance; a similar doublet with the yaw damper switched off produced a Dutch roll which was slow to subside, but was not divergent. All this says much for the success of Boeing's revised wing and the greater thrust of the new engines to drive it at M.082 - at a level 4,000ft higher than the 737-300's maximum of 37,000ft, where that aircraft would almost certainly be encountering Mach buffet at about M0.76. Throughout the manoeuvring at 41,000ft the cockpit was pleasantly quiet in terms of aerodynamic noise - a quality which Hewett attributes to the attention which has been directed at reducing sharp edges around the cockpit windows.

On the ground at Moses Lake the -700 proved to be very pleasant to taxi. The parking brake is released and set by a small lever at the rear of the power-lever quadrant, on the captain's side. A small amount of power moved the aircraft from a standstill, after which gentle use of the power levers and wheel brakes maintained taxi speed. (Boeing does not recommend the use of reverse thrust for speed control during taxiing). There is only one nosewheel-steering tiller - on the cockpit side wall by the captain's thigh - but a second can be specified by a customer.

Although the axis of operation for the tiller is more fore and aft than across the aeroplane (moving the tiller forward turns the aircraft to the right and vice versa), it comes easily to hand and is intuitive to use. The steering is nicely geared and friction free. The rudder pedals also have authority over about 7í of nosewheel deflection either side of centre although the steering tiller would always override the rudder pedals (for this reason, the tiller should be gripped during "full and free" checks of the rudder before take-off, to avoid scrubbing the nosewheel tyres).

The take-off at Moses Lake was from a 3,650m runway at a field elevation of about 150ft and a temperature of +25¹C. There was no significant surface wind, but there was a fair amount of thermal turbulence. Flap position 1 was selected; this is the first of eight possible flap selections on the -700 and results in some trailing-edge-flap extension, full extension of the leading-edge flaps and partial extension of the leading-edge slots. The relevant reference speeds were: V1 (take-off decision), 115 kt; Vr (rotate), 117kt; V2 (climb safety), 127kt. The technique for take-off is to advance the thrust levers to about 40% N1 and then press the take-off/go around (TO/GA) button on the left power lever.

The FADEC mode selected then appears as a legend at the top of the centre display panel above the primary engine indications. (The full list of modes is: TO - take off; CLB -climb; CRZ - cruise; G/A - go around and CON - continuous). The power levers were then advanced to take-off N1, while the control column was held with the left hand and the power levers guarded by the right hand until V1. The engines spooled up rapidly, and were brisk.

The aircraft was easy to keep straight using the rudder pedals - initially this system uses the rudder-pedal input to steer the nosewheels and then, of course, uses the rudder when it becomes aerodynamically effective (between 40kt and 60kt). Rotating the aircraft and establishing it in the climb by reference to the flight-director pitch command was easy with pleasant elevator-control forces. Retraction of the undercarriage and flaps in the climb produced no discernable pitch changes.

Before take-off, the auto-brake selector on the centre panel, near the undercarriage lever, had been set to RTO. This is the rejected-take-off position and had it been necessary to abort the take-off, closing the power levers at any ground speed above 90kt would have initiated full auto-brake; an abort before 90kt would have needed pilot-applied braking.

During manoeuvring at about 10,000-12,000ft, the -700 was a pleasure to handle. The controls were well harmonised in all axes and the forces pleasantly light for this size of aeroplane, while roll acceleration and rate of roll were good for an airliner. There are two hydraulic systems in the -700, either of which can operate all the primary flight-control surfaces to provide for hydraulic system redundancy. If both prime hydraulic systems were to fail, the ailerons and elevators could be operated manually, and the rudder by a standby hydraulic system.


Flight spoilers

There are four flight spoilers located on each wing upper surface, which are used differentially for roll control and symmetrically as airbrakes. Also on each wing there are two ground spoilers. A spoiler mixer, connected to the aileron cable drive, controls the hydraulic power-control unit on each spoiler to provide spoiler movement proportional to aileron movement. Pitch control is by elevators, and pitch trim by the variable-incidence tailplane.

Although the -700 was initially conceived as a derivative aircraft enjoying the 737 certification suitably amended, Boeing says that it eventually met all the certification requirements for a new type. Control-jam legislation is met by connecting the captain's roll control to the ailerons and the first officer's to the spoilers. Thus, whichever control became jammed, ailerons or spoilers, the remaining serviceable one could be operated by the relevant control wheel.

Each pilot's control column is connected to the elevator on his side of the aircraft; if a jam occurred, both pilots would apply a stick force to operate a splitting device between the elevators, allowing the surface not jammed to be operated by the control column on its side of the aircraft. Pitch control would then be less effective, but electrical trim operation of the tail plane would remain, thus allowing the high forces to be trimmed out. Finally, a Mach trim system operates above M0.615 by automatically adjusting the elevator angle to the tailplane through a Mach-trim actuator, which is driven by the flight-control computers.

Manually trimming the -700 longitudinally is done by using the double switches on either control column, which vary tailplane incidence via an electric motor. The rate of change is high with flaps extended and low with flap retracted. If the electric trim fails, trimming can be done manually using the trim wheels positioned one either side of the centre console.


Pitch-trim control

I liked the electric pitch-trim control, including the rate of change it produced both with flaps up or down. What I did not like was the whirring and clatter from the manual-trim wheels every time the electric trim operated (either through pilot operation or the autopilot electric-pitch trim when the autopilot was in use). This I believe is "classic" Boeing - I certainly remember it in the 727 - but I found it intrusive and out of place in an otherwise pleasantly quiet cockpit, to the extent that I sometimes had difficulty in hearing the intercomm or radio during the more prolific outbursts. Lateral trim is via a double electrical switch and directionally via a rotatable electric switch, both switches being mounted together towards the rear of the centre console.

During this phase of manual flying at or about 12,000ft, I again tried a stick slap to disturb the elevators and a rudder-pedal doublet to deflect the rudder. As before, the recovery was dead beat. Another doublet, this time with the yaw damper switched off, provoked Dutch roll, as at altitude, but, at this level, the roll was damped in some five or six cycles.

Next, trim change with flap extension and retraction was investigated, the selections being made at just below the limiting speeds in each instance. This took a little longer than it would on many aircraft because the -700 has such a plethora of flap settings.

The leading-edge devices consist, of, in total, four flaps and eight slats (two flaps inboard and four slats outboard of each engine). The slats extend to form either a sealed or slotted leading edge depending upon the extent of trailing-edge-flap deployment. These trailing-edge devices are double-slotted flaps inboard and outboard of each engine. When the flap lever is moved from UP to the 1, 2 or 5 position, the trailing-edge flap goes to a commanded position and the leading-edge flaps extend to the fully extended position, while the slats extend only part way.

When the flap lever is moved to the 10 degree, 15 degree, 25 degree, 30 degree or 40 degree position, the trailing-edge flaps move to their commanded position while the leading-edge flaps remain at the fully extended position, but the slats move to their fully extended position. Positions 1 to 15 provide increased lift, positions 15 to 40 increased lift and drag. Flap positions 30 and 40 are normal landing settings. Thankfully, the flaps are easier to use than to describe. Also their use, even close to the relevant limiting speeds, provoked commendably little pitch change. Flap selection is via a lever in a calibrated quadrant to the right of the power levers. The flap position selected is not always visible (at the lever) to the captain, dependent on the position of the power levers. The flap position gauge on the centre panel next to the undercarriage position lights is, however, clearly visible to both pilots.


Using the airbrake

Similarly, using the airbrake (speedbrake), selected via a lever on the captain's side of the power-lever quadrant, causes little pitch change but there is clearly evident aerodynamic burble.

Stalling the -700 was to prove a pleasure, because of its reassuring and predictable characteristics near and at the stall. There is a stick shaker (but not a stick pusher) and this was clearly needed because the natural stall characteristics were so benign. The natural stall warning was light buffet followed by a gentle G break which defined the stall and nose drop.

Power-off stalls were performed clean, with flap at position 15, gear down, and position 30, gear down; in the last two cases the nose rose to 15í before dropping. The aeroplane was held in the stall in the "dirty" configuration during which roll control remained good: there was no tendency to slice, nor for a wing to drop and the buffet remained soft throughout.

During a turn in the approach configuration with a little power on, an extreme nose attitude in a tight turn was required to provoke light buffet: relieving back pressure on the stick stopped the buffet and the aircraft was then rolled smoothly to wings level.

A failure of the right engine was simulated at 9,000ft with the aircraft in the take-off configuration, full power selected, at V2. The aircraft was easy to keep straight, using a little into-live-engine roll control, while rudder was applied towards the live engine and then held with a moderate foot force, which could in turn be easily trimmed out. The only difficulty in the whole test point was keeping the speed at V2 on both engines to be "on condition" for the simulated failure; the nose-up pitch angle was probably more than 20í, but progressively reducing that angle when using only the left engine to maintain V2 was not difficult.

After all this activity a restful, coupled descent and approach to the instrument-landing system (ILS) at Moses Lake was started. During the descent and marshalling phase, radar vectors were followed and a vertical-navigation profile was flown precisely and smoothly by the autopilot and FADEC. Hewett was happy for the ILS to be Category IIIA (automatic landing), even though not all the automatic landing trials for the -700 had been completed and the final autopilot gains had not been established. His confidence was well placed, however, as the precision approach was accurately and tightly flown, the flare was smooth and the touchdown gentle, despite the thermal turbulence.


Clear plot

Throughout the descent and approach, the map display had shown a clear plot of the aircraft's position and predicted track. Waypoints and traffic-alert and collision-avoidance system returns were shown on the map display and, when I chose to select the weather radar, the colour image did not clutter or reduce the clarity of the display.

Once the autopilot had placed the -700 on the runway, it was disconnected by pressing the controlwheel button to revert to manual flying. The aircraft was kept straight using the rudder pedals, Hewett set the flap and pitch trim and I advanced the power levers to the full-power end of the quadrant. Although this was a manual selection, the FADEC still monitored the engine parameters and would have trimmed out any tendency to exceed limitations. The -700 was extremely pleasant to fly in the visual circuit because of its good handling characteristics, ease of trimming, abundant power and excellent field of view.

On the second touch-and-go the same procedure was followed as for the first, but, at rotation, Hewett simulated an engine failure by moving the right power lever to flight idle. Once again, the -700 was easy and pleasant to fly during and after the simulated failure. The final approach was flown exactly as for a two-engined approach, except for the need to take out the rudder trim, set at about 200ft, before another touch-and-go using full power on both engines. On each landing, the aircraft was responsive, with excellent roll and pitch control to the point of touchdown allowing smooth arrivals to be made on the apparently compliant main gear.

At the end of this thoroughly enjoyable flight I had no doubts that Boeing had produced a competent version of the 737 in its -700. It handles extremely well, has excellent performance and many (but not all) of the operating benefits of modern technology.

Source: Flight International